Alternative thin film transistors for liquid crystal displays

ABSTRACT

Alternative thin film transistors for liquid crystal displays are disclosed. The alternative transistors can be used for panels of displays such as liquid crystal displays (LCDs), especially those having alternative pixel arrangements. These transistors can be oriented on a panel of an LCD using different, non-traditional configurations, while addressing misalignment and parasitic capacitance.

BACKGROUND

[0001] Thin-film transistor (TFT) misalignment and parasitic capacitancecan degrade the quality and performance of electronic devices such asliquid crystal displays (LCDs). One known attempt to correct for TFTmisalignments and any associated increase in parasitic capacitance isfound in U.S. Pat. No. 5,191,451 to Katayama et al (“the '451 patent”).FIG. 1A depicts the “double TFT” arrangement 100 of the '451 patent.Source line 104 connects to the TFT via source electrode 106. Two gateelectrodes 1708 are connected to gate line 102. Two drain electrodes 110connect to the pixel and are formed such that the two gate electrodes108 affect conduction from the source electrode to the drain electrodeswhen activated. It is noted that there are two crossover regions 112that are connected to TFT may produce additional parasitic capacitancebetween the gate and the source. As discussed in the '451 patent, anyvertical misalignment of the TFT placement is somewhat corrected by thisdouble TFT arrangement as is discussed therein.

[0002] Another manner of reducing the ill effects of TFT misalignment isshown in U.S. Pat. No. 5,097,297 to Nakazawa (“the '297 patent”). FIG. 4depicts a TFT 400 made in the manner taught in the '297 patent. As maybe seen in FIG. 2, gate line 402 delivers the gate signal to gateelectrode 408. Source line 404 sends image data to source electrodes406. When the gate electrode is activated, the image data is transferredto the pixel via the drain electrode 410. It is noted that this TFTembodiment contains only one gate crossover 412 which aids in reducingparasitic capacitance.

[0003] Furthermore, prior LCDs use the same orientation to aligntransistor in the pixel area of the dispaly. However, for alternativepixel arrangements, transistors may need to be located in unconventionallocations of a pixel area, while addressing misalignment and parasiticcapacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] The accompanying drawings, which are incorporated in, andconstitute a part of this specification illustrate exemplaryimplementations and embodiments of the invention and, together with thedescription, serve to explain principles of the invention.

[0005]FIG. 1 shows a prior art TFT having a double source/drainstructure.

[0006]FIGS. 2 and 3 show alternative TFTs having a double source/drainstructure.

[0007]FIG. 4 shows a prior art TFT with a double gate structure.

[0008]FIG. 5 show TFT structures in a reverse orientation and normalorientation, respectively.

[0009]FIG. 6 show TFT structures in a reverse orientation and normalorientation with an added gate crossover in the normal orientation tobalance any parasitic capacitance found in the reverse orientation.

[0010] FIGS. 7 show TFT structures in a reverse orientation and normalorientation with one fewer gate crossover in the reverse orientation tomatch any parasitic capacitance in the normal orientation.

[0011]FIG. 8 shows one novel pixel element design having a cornerremoved from the pixel to balance parasitic capacitances.

[0012]FIG. 9 shows yet another novel pixel element design havingmultiple corners removed to balance parasitic capacitances.

[0013]FIG. 10 shows yet another novel pixel structure in which at leastone extra line is added to shield the pixel element from parasiticeffects.

DETAILED DESCRIPTION

[0014] Reference will now be made in detail to implementations andembodiments, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

[0015] The following implementations and embodiments disclosealternative thin film transistors for liquid crystal displays aredisclosed. The alternative transistors can be used for panels ofdisplays such as liquid crystal displays (LCDs), especially those havingalternative pixel arrangements. These transistors can be oriented on apanel of an LCD using different, non-traditional configurations, whileaddressing misalignment and parasitic capacitance.

[0016]FIGS. 2 and 3 provide different alternative embodiments for theprior art double TFT structure shown in FIG. 1. These structures canprovide reduced source to gate capacitance, which can cause crosstalk incertain images. However, the gate to drain crossover can lessen thedamage to image quality. One advantage of the embodiment of FIG. 3 isthat there is only one crossover 132 that may reduce parasiticcapacitance.

[0017] Another set of TFT redesigns are shown in FIGS. 5 through 10 tohandle the unevenness of parasitic capacitance that might be introducedby the above described TFT remapping. As TFTs are remapped on the panel,it is possible for some TFTs on the panel to be implemented in differentcorners or quadrants of a pixel area. For example, some TFTs may beconstructed in the upper left hand corner of the pixel area, some in theupper right hand corner of the pixel area and so on. If all such TFTswere constructed the same way, then it would be likely that thesource-drain orientation would be reversed for left hand corner andright hand corner implementation. Such non-uniformity of constructionmight introduce uneven parasitic capacitance in the case of a given TFTmisalignment.

[0018]FIG. 5 is one embodiment of a TFT built with a reverse orientation502 as compared with a TFT built with a typical orientation 1904. Forexemplary purposes, TFT 504 is constructed within the upper left handcorner of its associated pixel in the usual manner—i.e. without anycrossovers to avoid any introduced parasitic capacitance. It is notedthat the source (S) and drain (D) electrodes are placed in aleft-to-right fashion. TFT 502 is shown constructed in the upper righthand corner of a pixel area in a reverse orientation—i.e. a crossover514 from source line 1906 is constructed so that the source electrode1910 and drain electrode 512 are also in left-to-right fashion. Thus, ifthere is a TFT misalignment in the horizontal direction, then TFTs 502and 504 will receive the same amount of added parasiticcapacitance—thus, keeping the panel's defects uniform. It will beappreciated that although TFT 502 and TFT 504 are depicted side-by-sideand connected to the same column, this is primarily for explanatorypurposes. It is unlikely that two adjoining subpixels would share thesame column/data line—thus, TFT 504 and its associated pixel is providedto show the distinction between a normal TFT orientation and TFT 502 ina reverse orientation.

[0019]FIG. 6 shows another embodiment of TFTs 602 and 604. As can beseen, a new crossover 606 is added to TFT 604 so as to balance the addedparasitic capacitance via crossover 604. FIG. 7 is yet anotherembodiment of TFTs 702 and 704. As may be seen here, the gate electrodecrossover 606 in FIG. 6 has been removed in favor of a gate linecrossover 706 which may have a lesser impact on individual pixelelements.

[0020]FIGS. 8 and 9 are embodiments of pixel elements with corners 810and 910 removed to match the one corner removed containing the TFTstructure. These pixel elements as designed here may balance theparasitic capacitances than a normal pixel structure.

[0021]FIG. 10 is another embodiment of a pixel structure that employs atleast one extra metal line 1010 that may help to shield the pixelelement from the parasitic capacitances between the gate lines and thepixel element. Additionally, if a dot inversion scheme is employed, thenthe opposing polarities on both lines 1010 will also help to balance anyparasitic capacitance between the source lines and the pixel elements.

[0022] Regarding the alternative TFT structures and pixel elementsdisclosed herein, standard LCD fabrication techniques can be implementedto form such structures. Moreover, the column, gate, and electrode linescan be formed of transparent material such as transparent conductiveoxide so as not to degrade the optical qualities of the LCD.

1. A device having a double thin film transistor, the device comprising:at least one drain electrode for the double thin film transistor; a gateline having at least two gate electrodes for the double thin filmtransistor; and a source line having at least one source electrode forthe double thin film transistor, wherein a crossover is formed by thedrain electrode and a gate line.
 2. A thin film transistor comprising: asource comprising a source electrode connected to a source line; a gatecomprising at least a first gate electrode and a second gate electrode,said first and second gate electrodes connected to a gate line; a drainconnected to said source electrode, said drain comprising at least afirst drain electrode and a second drain electrode; wherein there is atmost a single crossover between said gate and said source.
 3. The thinfilm transistor of claim 2 wherein there are at least two crossoversbetween said gate and said drain.
 4. A device comprising a plurality ofthin film transistors as recited in claim
 2. 5. A device comprising aplurality of first thin film transistors and a plurality of second thinfilm transistors, said first and said second thin film transistorscomprising a source electrode and a drain electrode; wherein said firstthin film transistors are substantally in normal orientation and saidsecond thin film transistors are substantially in reverse orientation;and wherein further said source electrodes and said drain electrodes ofboth said first thin film transistor and said second thin filmtransistor are oriented in substantially the same direction.
 6. Thedevice of claim 5 wherein said second thin film transistors comprise acrossover to effect substantially the same source electrode and drainefectode orientation as in said first thin film transistors.
 7. Thedevice of claim 5 wherein any misalignment of thin film transistors aresubstantially the same for said first thin film transistors and saidsecond thin film transistors.
 8. The device of claim 6 wherein saidfirst thin film transistors comprise a crossover to substantiallybalance any parasitic capacitance affected by said crossover in saidsecond thin film transistor.
 9. The device of claim 5 wherein saiddevice further comprises pixel elements connected to said first thinfilm transistors and said second thin film transistors; wherein furthersaid pixel elements have at least one corner removed.
 10. The device ofclaim 9 wherein said corner being removed is shaped substantially likethe corner comprising said thin film transistor.
 11. A pixel comprising:a thin film transistor, said thin film transistor connected to a gateline and a source line; a pixel element connected to said thin filmtransistor; and at least one extra metal line interposed between saidgate line and said pixel element.